Heat and Salt Changes on the Continental Shelf West of the Antarctic Peninsula Between January 1993 and January 1994

Hydrographic measurements from four cruises between January 1993 and January 1994 over the continental shelf west of the Antarctic Peninsula allow analysis of seasonal changes in heat and salt content of this region. Changes above the permanent pycnocline (about 150 m) follow a seasonal pattern of cooling and increasing in salt from summer to winter and warming and freshening from winter to summer. These near-surface changes, expressed as net heating or salting rate, were above 80 W m -2 and 4 mg salt m -2 s -1. The year to year difference was small compared to the seasonal changes. There was no seasonal pattern to the changes below the permanent pycnocline; heat and salt content increased or decreased together, with magnitudes about half (50 W m -2 and 2.0 mg salt m -2 s -1) that observed near the surface. Subpycnocline water warmed (10 W m -2) and increased salt (0.5 mg salt m -2 s -1) from one January to the next. Exchange of Upper Circumpolar Deep Water (UCDW), an oceanic water mass, and West Antarctic Peninsula modified Circumpolar Deep Water, a cooled version of UCDW on the shelf, is responsible for these changes. During the exchange process, UCDW cools by loosing heat to the cold, near-surface Winter Water left by the deep mixing during the previous winter. Subpycnocline heat and salt changes occur as a difference between onshore and vertical diffusion with vertical diffusivities of (1.0, 0.36) x 10 -4 m 2 s -1, for heat and salt, respectively, and a horizontal diffusivity of 37 m 2 s -1. The annual change is due to a net flux of UCDW onto the shelf during 1993, with most of the exchange occurring fall and winter. Meandering of the Antarctic Circumpolar Current along the shelf break in this area seems to cause these exchanges. Deep across-shelf cuts in the bottom topography may also play a role.